Hard to machine materials, such as titanium and its alloys, produce localized extreme temperatures during machining. This limits cutting efficiency and also quickly wears out expensive tools. Short tool life leads to frequent interruptions in manufacturing, high maintenance costs, and sometimes damage to an expensive workpiece being machined. Damage to a workpiece increases manufacturing defect rates and raises expense overall.
Titanium alloys are often used to produce complex and critical parts used, for example, in aircraft and medical implants. Additional example applications include aerospace structures and engines, rockets, spacecraft, turbines, automotive engine components, nuclear and chemical plants, petrochemical industries, offshore engineering, food processing, and biomedical devices. The alloys possess high strength-to-weight ratio, high-temperature strength, strong fracture and corrosion resistance, and biocompatibility.
Titanium alloys are very difficult to machine, however, and tool life is poor in systems that machine titanium. Titanium has poor thermal conductivity and low elongation-to-break ratio. Titanium is also chemically reactive with typical tool materials at a cutting temperature of 500° C. and above. As a result, highly-localized temperatures are developed at the tool-chip interface. Severe edge chipping and plastic deformation via galling and seizure of chips are often produced. This ultimately shortens tool life, can be detrimental to surface finish, and can cause parts to fail quality requirements.
Various efforts have been made to address these problems in machining titanium. One technique is known as flood cooling. See, e.g., Nandy, A. K., et al., “Some studies on high-pressure cooling in turning of Ti-6Al-4V,” International Journal of Machine Tools and Manufacture, 49: 182-198 (2009); Cheng, C., et al., “Treatment of spent metalworking fluids,” Water Research, 39: 4051-4063 (2005). The flood techniques are used in practice, despite relatively ineffectiveness and also unfriendliness to the environment due to large quantities of toxic fluids used for cooling/lubrication.
High pressure cooling technique applies coolant at 70-160 bar or more directly at the tool/workpiece interface. A three to four-folds tool life increase compared to flood cooling has been reported by some. See, e.g., Nandy & Paul, “Effect of coolant pressure, nozzle diameter, impingement angle, and spot distance in high pressure cooling with neat oil in turning Ti-6Al-4V,” Machining Science and Technology, 12: 445-473 (2008); Palanisamy, S., et al., “Effects of coolant pressure on chip formation while turning Ti6Al4V alloy,” International Journal of Machine Tools and Manufacture, 49: 739-743 (2009). In practice however, overall productivity improvements have been reported to be about 50%. The lower productivity improvement is attributable to a higher consumption rate of the cutting fluid, its delivery cost at such high pressure, and the system setup cost. Pusavec, F., et al., “Transition to sustainable production-Part I: application on machining technologies,” Journal of Cleaner Production, 18: 174-184 (2010).
Another difficult to implement process is cryogenic cooling. While offering improved tool life, this is an energy-intensive process that requires liquid nitrogen (LN2) to be delivered at high rates in the range of about 45-250 L/hr. Hong, S. Y., et al., “New cooling approach and tool life improvement in cryogenic machining of titanium alloy Ti-6Al-4V,” International Journal of Machine Tools and Manufacture, 41: 2245-2260 (2001). The liquid nitrogen delivery also poses safety risks to operators and other personnel.
With a goal of environmental friendliness, others have used supercritical CO2 (scCO2) as a solvent to dissolve cutting fluid. Clarens et al., “Evaluation of cooling potential and tool life in turning using metalworking fluids delivered in supercritical carbon dioxide,” Proc. of the ASME International Manufacturing Science and Engineering Conference (MSEC), October 4-7, West Lafayette, Ind., USA (2009). In this method, CO2 gas is provided at levels substantially above its critical pressure, 72.8 bar. Tool wear rates realized during micro-machining were approximately equal to those of conventional flood emulsion systems. In experiments described in this paper, scCO2 spray was provided at 130 bar. These high pressures required a heavy and sophisticated system layout. The costs are prohibitive for such a system, given the lack in improvement over the flood techniques. Also, high pressures pose safety risks to operators and the other personnel. Finally, only low cutting speeds of ˜45 m/min and depth of cut (0.27 mm) was reported, which would not be well-suited for macro-machining applications.
Efforts by some of the present inventors and colleagues have focused on atomized spray application of cutting fluids, and have proven to be successful in micro-machining applications. Micro-machining of AISI 1018 steel with atomized cutting fluid droplets was demonstrated in Jun, Joshi, DeVor, and Kapoor, “An experimental evaluation of an atomization-based cutting fluid application system for micromachining,” ASME Transactions—Journal of Manufacturing Science and Engineering, 130: 0311181-8 (2008). This system was limited to a flow rate of about 1 mL/min, which is ill-suited toward macro-machining applications in general, and also toward the more difficult materials, such as titanium alloys. Macro-machining applications require machining at or above about 1 mm depth of cut and 0.1 mm/rev or higher feed rate. This larger cutting zone creates faster evaporation rates and, in the disclosed set-up, a small amount of cutting fluid can even evaporate prior to reaching the tool-workpiece interface.
Typical commercial nozzle units used in minimum quantity lubrication (MQL) systems employ a high-velocity gas to produce fluid droplets with shear mechanism. The size of fluid droplets varies in a wide range in such systems. The fluid flow rate in these systems is typically limited at ˜2-3 mL/min, a level that is insufficient for providing cooling and lubrication effect during machining at the macro-scale.
Machining of difficult materials, especially of materials having properties like titanium alloys, and especially at the macro-machining level, remains inefficient and expensive. Tools are replaced often and machine surfaces can exhibit defects. Defects can compromise part integrity and can cause a high part rejection rate, leading to additional expense.